Many technologies have been used to perform desalination resulting in preferred technologies based on economics. For example, in the desalination of mild brackish water having less than 1000 ppm Total Dissolved Solids (“TDS”), Reverse Osmosis (“RO”) is a preferred desalination technology. This is a reflection of the fact that other technologies involve phase change (such as boiling) whereas RO employs low-pressure pumps (less than 100 psia or 7 bar) to force water through semi-permeable membranes resulting in less energy consumption than that involved in a phase change process. However, in purification and/or treatment of waters containing non-filterable suspended particulates, RO is ineffective. For example, water from the Colorado River contains silt in the 1 micron range, which tends to foul RO membranes, thereby increasing the maintenance and/or pretreatment costs of RO operation. Similarly, chemical-mechanical-planarization slurries containing less than 1 micron silica foul membranes.
For aqueous applications for liquids with higher TDS such as RO concentrates, waste streams, and seawater, other mechanical and thermal technologies economically compete with RO. In the case of seawater desalination, the RO pump pressures increase to 1200 psia, or 80 bar, and feed waters require expensive pre-treatment in order to protect and extend the life of the RO membranes.
Technologies competitive with RO for seawater desalination include Mechanical Vapor Compression (“MVC”), Multi-Stage Flash Distillation (“MSF”), and Multi-Effect Distillation (“MED”) with and without Thermal Vapor Compression, among others. The MVC needs shaft power to drive its compressor. The motor can be either electrically or thermally driven. For electrically driven MVC, MVC plants consume more electricity than RO units in the same seawater service. The other processes dominantly use and reuse heat as the main driver to affect temperature-driving force between boiling and condensing at staged pressures. The thermally driven plants attempt to reuse the high temperature applied heat as many times as is economically possible to minimize operating costs. This energy reuse factor economically varies from 6 to 12.
Thermal processes that operate below the boiling point of water are called Humidification/De-Humidification (“HDH”). Certain HDH units require two heat transfer towers (or zones) to transfer heat from a massive flow of water. The water is used as both an internal heat source and internal heat sink. The requirement of two towers makes the HDH process energy inefficient. Accordingly, improved systems and methods for desalination remain desirable.